Feline Tritrichomonas foetus Infection

Tritrichomonas foetus was known as a parasite of the reproductive tract of cattle for many decades prior to its implication in feline large bowel diarrhea.¹ In the mid-1990s, trichomonads were noted in the feces of kittens experiencing colitis; these organisms were molecularly identified as T foetus in the early 2000s.^2-5 It has since been determined that the T foetus affecting felines is a distinct genotype from that affecting bovine hosts.^6-9 Some researchers have suggested that the feline genotype represents a separate species and have proposed the name Tritrichomonas blagburni.¹⁰

Clinical Signs

In cats, T foetus primarily colonizes the cecum, with the distal ileum and descending colon being colonized to a lesser extent. Similar to Giardia duodenalis, these organisms are noninvasive and remain within the mucus layer and in contact with the surface epithelium of the gastrointestinal tract.¹¹ Infection produces lymphoplasmacytic and neutrophilic inflammation, resulting in chronic colitis.¹¹ Clinically affected cats exhibit foul-smelling large bowel diarrhea, frequently containing mucus and/or blood.¹² In experimental infections, clinical signs occur 2 to 7 days following ingestion of trophozoites.⁴ In severe cases, inflammation of the anus and rectal prolapse may occur.¹³ Cats may experience diarrhea for years, although it is common for clinical signs to be intermittent.¹⁴ A temporary response to antibiotic treatment followed by recurrence of clinical signs is a common clinical feature of disease.¹⁵

Infections and clinical signs are most common in young cats, with the median age of diagnosed cats being 1 year.¹⁴ Infection is often associated with high-population density situations such as catteries and shelters.¹² Asymptomatic infections occur and may serve as a source of infection for other cats.¹²

Life Cycle

The life cycle of T foetus is straightforward and involves only 1 life stage—the trophozoite. Trophozoites divide by binary fission within the host and are shed in the feces. Transmission between cats is direct, via the fecal–oral route. A common scenario is exposure of susceptible cats via shared litter boxes, with ingestion of trophozoites occurring during the grooming process.^16,17 Trophozoites can survive for several days in moist feces but are not long-lived under clean, dry environmental conditions.¹⁸ Trophozoites are susceptible to many common cleaning agents, including ethanol and bleach.¹⁹

Diagnosis

Differentials for the clinical presentation associated with T foetus infection in cats include other intestinal parasites such as Giardia, Cryptosporidium, and Cystoisospora. Diagnosis of feline trichomonosis can be made via direct fecal examination, culture, or PCR testing.

Fecal Examination

Direct fecal examination as a diagnostic method for feline T foetus infections has low sensitivity but is simple and inexpensive to perform. The specificity of this test depends on the individual examining the slide and their ability to differentiate T foetus from other flagellate protozoans such as Giardia species and Pentatrichomonas hominis.

T foetus trophozoites are pear-shaped, 10 to 25 µm in length, and 3 to 15 µm in width.²⁰ They possess 3 anterior flagella—hence the genus name—and a single posterior flagellum, which originates from an undulating membrane that spans the majority of the length of the organism (Figure 1). The movement of T foetus is described as jerky, although the organisms do exhibit forward motility. The jerky movement of T foetus should be differentiated from the “falling-leaf” movement of Giardia organisms.¹²

Figure 1. Tritrichomonas foetus trophozoites are pear-shaped with 3 anterior flagella (2 are visible in this image) and a single posterior flagellum that originates from an undulating membrane.

The sensitivity of direct fecal examination for T foetus diagnosis has been reported to be as low as 4% in naturally infected cats; sensitivity can be maximized by using fresh, nonrefrigerated diarrhetic feces.^18,21,22 Fecal flotation to rule out Cryptosporidium, Cystoisospora, and Giardia infections should be included in the workup of these cases, with zinc sulfate being the flotation solution of choice for detecting Giardia.

Fecal Culture

Fecal culture to enrich trophozoites can be performed using the commercially available InPouch TF Feline test pouches (Biomed Diagnostics) or modified Diamond’s trypticase–yeast extract–maltose media.¹⁸ To perform trophozoite culture, media should be inoculated with a small amount of feces (grain of rice) and incubated at room temperature or 37 °C (98.6 °F).¹² Cultures should be maintained in the dark, and pouches should be in an upright position.

Cultures can be observed daily using light microscopy for the presence of organisms. Trophozoites are typically observed within 72 hours when cultures are incubated at 37 °C, while room temperature–incubated samples may require up to 12 days of culture to reach observable numbers.¹² The number of organisms present in the fecal inoculum affects the incubation time required to reach observable numbers. It is recommended that fecal culture for T foetus be performed in-clinic and the inoculated media not be shipped for evaluation, as fluctuating temperatures during shipment may affect the results.¹²

While Giardia organisms do not survive beyond 24 hours within the InPouch system, P hominis can proliferate within the pouches.¹² Careful morphologic evaluation or molecular methods should be employed to differentiate T foetus from P hominis.

PCR Testing

Many T foetus PCR protocols have been published, and testing is widely available at diagnostic laboratories throughout the country. PCR is considered the most sensitive and specific diagnostic test for T foetus infections in cats. Sample collection and submission guidelines may vary between laboratories and should be followed closely to ensure accurate test results. PCR testing allows for differentiation between T foetus, P hominis, and Giardia species.

Histopathology

Trophozoites can be observed histologically in colonic biopsy samples, but the sensitivity of histopathologic identification as a diagnostic method is considered quite low.¹¹ Immunohistochemistry and in situ hybridization are used in research but are not generally available for diagnostic use. Histopathologic evaluation can aid in ruling out or deprioritizing inflammatory bowel disease as a cause for chronic diarrhea in cats.

Treatment

Following diagnosis, efforts should be made to prevent continued exposure and reinfection. Cleaning and disinfection of the litter box and other surfaces should be performed frequently to eliminate trophozoites.

Treatment options for feline trichomonosis are limited, and response to treatment is inconsistent (Table 1). Nitroimidazole compounds, including metronidazole, ronidazole, and tinidazole, have been used to treat T foetus infections in cats.²³ Ronidazole is currently considered the treatment of choice; however, this compound is not always available, and there are reports of ronidazole-resistant infections.²⁶ In instances when ronidazole is unavailable, tinidazole is the recommended treatment, although it is less efficacious.²⁵ Neither of these products is labeled for use in cats or for the treatment of T foetus infections in any host. Metronidazole demonstrates minimal efficacy against feline T foetus infections.^27,28

The recommended dose for ronidazole (30 mg/kg/day) is the highest dose that can be safely administered to cats. Reversible neurotoxicity occurs in some cats.^12,14,24 If neurologic signs occur, the treatment should be stopped. If a compounding pharmacy is used as a source for ronidazole or tinidazole, the prescribing veterinarian should ensure the pharmacy is reputable, as dosing errors can lead to adverse effects, including neurotoxicity.

It is estimated that 60% of cats treated as recommended with ronidazole will have near-complete resolution of clinical signs, although they may remain infected.¹² Failure of clinical signs to resolve may be a result of inaccurate diagnosis, underdosing of ronidazole, reinfection, or infection with a ronidazole-resistant population of T foetus. The prevalence of ronidazole-resistant T foetus is unknown but is estimated to be significant.²⁶

It can be challenging to determine if resolution of diarrhea following treatment is a result of true infection clearance or simply concealment of the infection. In cases where the true infection status is important (e.g., prior to reintroduction to a multicat environment), post-treatment PCR testing on a colonic flush sample is recommended.¹²

Summary

Feline T foetus infections are a common cause of chronic colitis, especially in young cats and cats in high-density populations (e.g., catteries, shelters). Intermittent, foul-smelling diarrhea containing blood and mucus is the most common clinical presentation. Asymptomatic infections occur and likely serve as a source of infection for susceptible cats. Diagnostic options include direct fecal examination, trophozoite culture, and PCR testing, each with its own benefits and limitations. Ronidazole is the treatment of choice, but it is not always available and treatment is often unrewarding.